In a wavelength division multiplexing (WDM) optical transmission system, optical signals at a plurality of wavelengths are encoded with digital streams of information. These encoded optical signals, or optical channels, are combined together and transmitted through a series of spans of an optical fiber comprising a transmission link of a WDM fiberoptic network. At a receiver end of the transmission link, the optical channels are separated, whereby each optical channel can be detected by an optical receiver.
While propagating through an optical fiber, light tends to lose power. This power loss is well understood and is related to the physics of propagation of light in the fiber. Yet some minimal level of optical channel power is required at the receiver end to decode information that has been encoded in an optical channel at the transmitter end. To boost optical signals propagating in an optical fiber, optical amplifiers are deployed at multiple locations, known as nodes, along the transmission link. The optical amplifiers extend the maximum possible length of the link, in some instances, from a few hundred kilometers to several thousand kilometers, by amplifying optical signals to power levels close to the original levels of optical power at the transmitter end.
An erbium-doped fiber amplifier (EDFA) is one of the most practical types of optical fiber amplifiers employed in many modern fiberoptic networks. A single EDFA module can amplify up to about a hundred of optical channels at a time, thus providing significant cost savings. One of the main components of an EDFA is a length of an amplifier optical fiber having a core doped with ions of a rare earth element erbium. One EDFA module can contain two or more erbium doped fibers. Each erbium doped fiber (EDF) is optically pumped by a semiconductor laser, so as to create a population inversion between energy states of the erbium ions comprising a gain medium of the EDF. Once the population inversion is created, the gain medium begins to amplify an optical signal propagating along the core of the EDF. The optical signal comprises a plurality of individual optical channels. The gain medium is characterized by a wavelength-dependent gain coefficient, from which amplification coefficients of these optical channels can be determined. During the amplification process, the optical power of the pump is absorbed by the gain medium, which simultaneously amplifies all the optical channels present. Therefore, the amplification coefficient of a particular channel depends on the optical power and the number of optical channels present, and on the optical power of the pump. When the number of optical channels changes due to switching and routing of some optical channels, the gain coefficient of the rest of the optical channels changes, usually in the form of a spectral tilt of the gain coefficient. At the same time, a goal of the amplifier is to provide constant gain, which should not depend on the power or wavelength loading condition; otherwise, some channels will not have sufficient power and signal-to-noise level at the receiver end, resulting in information being lost.
The control electronics of EDFAs partially solves the problem of the variable signal load. More particularly, the total optical power at the input and at the output of the amplifier is measured, and the average gain coefficient of the amplifier is calculated as the ratio of the output optical power to the input optical power. The amplifier control electronic circuitry adjusts the amplifier's pump powers through a feedback loop in such a way that the measured optical gain coefficient equals to the desired or “set” optical gain coefficient and is not varied significantly in time. The feedback loop also compensates for a gradual reduction of semiconductor laser pumping efficiency and a gradual reduction of EDF amplification coefficient over the lifetime of the amplifier.
Over the last few years, the performance of EDFAs has been improved mostly due to improvement of performance characteristics of individual components such as pump lasers, optical couplers, and the like. Control software and the EDFA control electronics have also been improved. Further improvement of EDFA performance is very difficult without increasing the number of amplifier components. Increasing component count has been hindered by a substantial downward market pressure on EDFA cost and size.
Despite this downward market pressure, the technology base of an EDFA has not changed substantially. EDF remains the gain medium, semiconductor lasers are used to pump the EDF, and discrete fiber-coupled components such as optical taps and WDM couplers, optical isolators, gain flattening filters, and variable optical attenuators, are still used to properly couple and guide signal light and pump light. Fiber-coupled photodiodes are used to measure input and output optical power levels. Fiber splicing is used to optically couple the components together. As a result, a typical prior-art EDFA has numerous splices, splice protectors, discrete components, and optical fiber loops. The multitude of components and fiber loops make EDFAs of the prior art complex and costly. Using prior-art technologies and approaches, it is possible to further improve performance by substantially increasing cost of an EDFA, which is prohibitive in the present market environment. It is also possible to further reduce amplifier costs by sacrificing EDFA performance characteristics such as the spectral gain tilt, flatness of the gain spectrum, and the noise figure of the EDFA, which is undesirable from the standpoint of maintaining a high level of technical performance.
It is therefore a goal of the present invention to provide an EDFA that allows for reduction of size, complexity, and cost without compromising the optical performance. Correspondingly, it is also a goal of the present invention to provide an EDFA that allows addition of new components and further optical performance improvement at a small incremental cost, as compared to the cost of equivalent discrete components.
An EDFA of the present invention meets the above stated goals. New components can be added to an EDFA at a small incremental cost. By using the present invention, it is possible to improve EDFA characteristics and, or introduce additional functionality with minimal cost and size increases.
Furthermore, the present invention facilitates usage of new integrated EDFA components. These new components further improve performance and reliability of an EDFA of the present invention and enable more efficient control of key EDFA performance characteristics, while allowing for a considerable size and cost reduction.